1. Technical Field
The present invention relates to an illuminating device, such as a backlight unit used for, for example, a liquid crystal display device of a cellular phone.
2. Related Art
A liquid crystal display device includes a backlight unit on the rear surface side of a liquid crystal display panel to perform transmissive display. In general, the backlight unit is composed of an illuminating device including a light source, an optical waveguide for emitting light from the light source to the rear surface of the liquid crystal display panel as surface light, a sheet for diffusing light emitted from the optical waveguide, and a prism sheet for condensing light. Light incident on the optical waveguide from the light source travels in the optical waveguide while repeatedly reflecting from an emission surface and a reflective surface of the optical waveguide, and is then emitted from the emission surface to the outside.
The light emitted from the optical waveguide is incident on the prism sheet. The prism sheet refracts the incident light to change the traveling direction of light. In this way, the prism sheet can emit light toward the liquid crystal display panel. JP-A-7-174910 discloses a prism sheet formed of a material having a high refractive index. The prism sheet having a high refractive index can refract light at a large angle to make it incident on the liquid crystal display panel in the vertical direction, resulting in an improvement in the brightness of an illuminating device.
However, the optical waveguide emits light mainly in a specific direction light, rather than uniformly in all directions. Therefore, merely increasing the refractive index of the prism sheet without taking the characteristics of light emitted from the optical waveguide into consideration is insufficient for improving the brightness of an illuminating device.